Flexible hydraulic and pneumatic hose assemblies have been used in a variety of fluid transfer applications including, for example, in aircraft engines. Hose assemblies for various applications must often be able to sustain high pressures over relatively long periods of time. It is also desirable for such hoses to have good abrasion resistance, low leakage of fluids, good mechanical strength, low permeability to moisture and/or vapors, good strength to weight ratio, good corrosion resistance, suitable flexibility, and the like.
In some environments, such as in aircraft or aerospace applications, hoses may be subjected to high temperatures. For example, hoses may be utilized in areas where there may be a danger of fire. Thus, a hose assembly may require fire resistant protection to withstand the high temperatures encountered in a fire to minimize the likelihood of fire or high temperatures damaging the hose. FAA standards require certain devices to withstand the heat of a burning aircraft for a period of fifteen minutes before exposing the enclosed devices to fire. For example, a fuel hose must not release fuel into a fire for the initial fifteen minutes of burning. Under the current standards after fifteen minutes have elapsed, it is no longer necessary to prevent the flame from reaching the covered device.
Hoses having fire resistant properties are known in the art. For example, it is known to encompass the hose and portions of the associated hose fittings with a fireguard or firesleeve. The firesleeve often consists of a silicone rubber tube surrounding the hose and/or fitting. Silicon rubber firesleeves, however, may ablate when subjected to fire. Silicon rubber firesleeves also do not protect the hose from oil, fuel, or chemical attack, and also do not provide resistance to chafing. In some instances, a metal tube has been provided as an outer layer of a firesleeve surrounding an insulating layer such as a silicone layer. At temperatures above 450° F., such protective covers may only provide suitable protection for a relatively short period of time. In particular, silicon rubber firesleeves tend to dry, shrink, and crack when exposed to extremely high temperatures for extended periods of time, and may fail at around 14,000 hours.
The ability of a hose to withstand high temperatures for a given period of time may also be affected by the fluid flow through the hose. In the past, maintaining a fluid flow in the hose (such as from the fluid being transferred through the hose) was desirable to keep the underlying hose cool during a fire. The fluids being transferred through the hose, however, are typically flammable. Thus, a hose failure could result in flammable fluids coming in contact with the fire, which may feed or intensify the fire. Consequently, there has been a trend in the aircraft industry to build engines in which the flow through hoses may be reduced or stopped in the event of a fire.